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Abstract

Radial breathing oscillation of 2-D arrayed GaN nanorods was successfully excited in rods with different diameters by using femtosecond transient reflectivity measurement. Through analyzing thus measured diameter dependent oscillation frequency, we discovered that modification of the mechanical property appeared in the 2-D arrayed piezoelectric GaN nanorods, fabricated on top of a bulk substrate, when the rod diameter was on the order of or less than 50 nm. Our measurement observed a much reduced elastic stiffness constant (C11) of 193 ± 24 GPa in 35nm diameter nanorods, compared with the 365 ± 2 GPa in bulk GaN. This size-reduction induced mechanical modification would be a critical factor to be considered for future sensing and energy applications. Our study also provides a new spectroscopic method to explore the size-reduction-induced softening effect through the measurement of the radial breathing oscillations.

(a) Measured probe reflection change as a function of time delay. The sample is with a 135nm diameter and a 220nm period. The inset scheme shows the geometry of a nanorod. D, d1, d2 and L are average diameter, top diameter, bottom diameter and height of a nanorod, determined by top and side SEM views. AR: aspect ratio. (b) An oscillation frequency of 33GHz was revealed in the fast-Fourier-transformed spectrum. The inset figure illustrates the observed oscillatory signal after removing the carrier dynamics background of (a).

(a) The observed oscillatory frequency versus the diameter of nanorod. Rod diameter ranges from 324nm to 35 nm. The horizontal error bars represent the diameter nonuniformity based on the SEM images and the vertical error bars represents full-width half-maximum of the amplitude in Fourier-transformed oscillatory spectra. The black curve is the result of the theoretical calculation on the radial breathing mode based on Eq. (1) with vr = 7744 m/s and τn = 2.05. (b) Elastic stiffness constant (C11) as a function of GaN rod diameter. The black dashed line represents the bulk C11 value.